Detecting volcanic ash in jet engine exhaust

ABSTRACT

Onboard systems and methods for early detection that an aircraft has flown into a volcanic ash plume embedded within a water vapor cloud having a concentration of a volcanic ash which would be dangerous to an aircraft. The detection method generally comprises the steps of measuring the infrared emission characteristics of a jet engine exhaust and generating a detection signal when the intensity of infrared emissions at or near a spectral peak wavelength exceeds a threshold.

TECHNICAL FIELD

This disclosure generally relates to systems and methods for sensingwhen an aircraft is encountering a volcanic ash plume and, moreparticularly, to systems and methods for onboard detection of volcanicash plumes embedded in water vapor clouds.

BACKGROUND

As used herein, the term “volcanic plume” or “volcanic ash plume” meansa cloud of volcanic ash, the term “volcanic gases” means gases given offby active volcanoes, and the term “gas plume” means a plume of avolcanic gas. Dispersed volcanic gases disposed outside the volumeoccupied by a volcanic ash plume are not included as part of the“volcanic ash plume” as the latter term is used herein.

Volcanic ash can pose a hazard to flying jet aircraft, threaten thehealth of people and livestock, damage electronics and machinery, andinterrupt power generation and telecommunications. Volcanic ashcomprises tiny jagged particles of rock and natural glass blasted intothe air by a volcano. Wind can carry ash thousands of miles, affectingfar greater areas than other volcano hazards.

Volcanic plumes present two problems for aircraft: (a) enginemalfunction due to ash; and (b) aircraft damage and/or crew andpassenger injury due to ash and corrosive gases. Volcanic ash particlesare extremely abrasive. They are jagged particles of rock and glass thatcan cause rapid wear to the internal workings of jet engines. Moreimportant, high temperatures in some parts of jet engines can melt ashthat is passed through an engine. The ash then re-solidifies on coolerparts of the engine, forming a layer that blocks airflow, interfereswith moving parts, and eventually causes malfunction of the engine. Itis therefore desirable for aircraft to be capable of detecting volcanicash prior to encountering the ash or as quickly as possible thereafterto avoid prolonged exposure to the ash.

Various known solutions for detecting and avoiding a volcanic plumeduring flight of an aircraft have certain disadvantages. First, forvolcanoes that are well monitored, sensors or people on the ground canquickly observe an eruption and report it to flight safety authoritiessuch as the FAA. In these cases, a notice to airmen is issued. However,many remote volcanoes around the world are still not well instrumentedand can erupt without immediate detection. Even after detection, themechanism to issue a notice to airmen imposes a delay for processing anddistribution, during which an unwarned aircraft may encounter the plume.

Second, a few satellites are capable of detecting volcanic plumes fromorbit, based on the sulfur dioxide spectra, the thermal infraredemission, visible ash clouds, or a combination of these. When asatellite detects a volcanic plume, a notice to airmen is issued.However, satellite observations are not continuous. An eruption thatoccurs between satellite passes may go undetected for 6 to 12 hours,which is more than enough time for aircraft to encounter the plume. Theperiod of non-detection may go on longer for small eruptions or duringovercast conditions. Even after detection, the mechanism to issue anotice to airmen imposes a delay for processing and distribution, duringwhich an unwarned aircraft may encounter the plume.

Third, in daytime clear weather, pilots can visually observe adistinctive volcanic plume and avoid it. Visual observation may be donewith the naked eye or with cameras using natural light, infraredemission, optical backscatter measured via lidar or optical backscatterusing standard aircraft light sources. Airborne ash particles areexposed and able to reflect light or emit infrared radiation. However,volcanic plumes are often encountered during nighttime and/or embeddedwithin other clouds, such as meteorological clouds containing waterdroplets or ice crystals, rendering visual detection methodsineffective. In this description, water droplets and ice crystals willbe referred to collectively herein as “water precipitate” or“precipitate” for conciseness, and the term “precipitate particles” willrefer to droplets of water or crystals of ice embedding ash particles.Meteorological clouds not only surround a volcanic plume, but, becausethe individual ash particles serve as nucleation sites for precipitateparticles, the individual ash particles become embedded in precipitateparticles. Therefore, the ash particles are not visible and contributealmost nothing to the electromagnetic signature of the cloud.

Typical uses of infrared emission to detect volcanic plumes use sensorsdirected toward the natural atmosphere. For example, U.S. Pat. No.5,654,700, entitled “Detection System for Use in an Aircraft,” proposesa system that would detect a volcanic ash cloud ahead of an aircraft bymonitoring infrared radiation that traverses the ash cloud. However, theoptical and infrared signatures of ash particles that are embedded inprecipitate particles are camouflaged and remain hidden from suchinfrared sensors.

If volcanic ash is not detected, the first sign to an aircraft crew thatthe aircraft has flown through a water vapor cloud containing volcanicash is engine failure. A typical pilot response when an engine begins tofail is to increase power. However, when volcanic ash is present, thiscould make the situation worse. If ash is suspected as the cause of anengine failure, then a pilot may throttle back engines, turn on engineand wing anti-ice devices and lose height to drop below the ash cloud assoon as possible. This action typically helps to restore enginefunctionality. However, because the aircraft may have already flownthrough a substantial amount of ash, aircraft parts may have alreadysuffered costly damage which may require maintenance, repair orreplacement of engine parts. Therefore, avoiding any amount of flighttime through ash helps to reduce any potentially damaging effects of theash, and therefore helps to save maintenance time and money.

There exists a need for a system that will detect volcanic plumesembedded in clouds and ash particles embedded in precipitate particles,and alert an aircraft to avoid such volcanic plumes, or to rapidlychange course to escape such volcanic plumes.

BRIEF SUMMARY

The foregoing purposes, as well as others that will be apparent, areachieved generally by providing a detection system installed onboard anaircraft for detecting volcanic ash in jet engine exhaust and alertingthe pilot upon detection of volcanic ash emitted from the jet engine.Such a system provides for reliable detection of volcanic ash particleseven when they are embedded in a water vapor cloud.

The onboard system comprises one or more infrared sensors positioned onthe aircraft to face downstream along the jet engine exhaust, andconfigured to discriminate ash emission from the normal infraredemission of jet engine exhaust. The sensor's field of view includesatmosphere that has been heated by passage through a jet engine. Theheat of the engine evaporates the precipitate particles, exposing theembedded ash particles to detection by the infrared sensor.

The sensor is configured to measure the thermal infrared emission fromthe jet engine exhaust, and detect an anomalous rise therein. Thedetection system then generates a detection signal when the anomalousrise in thermal infrared emission exceeds a user-specified orpre-determined threshold.

Other aspects of the invention are disclosed and claimed below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram (not to scale) showing an aircraft flying towards awater vapor cloud containing embedded particles of volcanic ash.

FIG. 2 is a diagram showing a close-up of the inside of a water vaporcloud containing particles of volcanic ash embedded in precipitateparticles.

FIG. 3 is a diagram showing a system for detection of volcanic ashparticles, installed on an aircraft that has flown through a water vaporcloud containing embedded particles of volcanic ash and which is leavinga trail of jet exhaust containing heated and exposed volcanic ashparticles.

FIG. 4 is a block diagram of a system for detection of volcanic ashparticles embedded in a water vapor cloud.

FIG. 5 depicts steps in a method for detection of volcanic ash particlesembedded in a water vapor cloud.

FIG. 6 is a graph showing estimated infrared emissions detected by theinfrared sensors located and oriented as described herein.

FIGS. 7A-7D depict additional jet engine configurations and exemplaryplacement and orientation of sensors for detecting ash appropriate forthose jet engine configurations.

Reference will hereinafter be made to the drawings in which similarelements in different drawings bear the same reference numerals.

DETAILED DESCRIPTION

The present disclosure provides systems and methods for early detectionof volcanic ash embedded in clouds. In FIG. 1, aircraft 100 is depictedflying towards a water vapor cloud 102 having volcanic ash 104. In FIG.2, a close-up of the water vapor cloud 102 is shown, in which particlesof volcanic ash 202 are embedded in water droplets 204. Althoughdescribed below as embedded in water droplets 204, volcanic ashparticles 202 may also be embedded in crystals of ice.

Detection of volcanic ash in water vapor clouds using the disclosureprovided herein is done after an aircraft 100 has already flown throughat least a portion of a water vapor cloud 102 containing volcanic ashparticles 202, as shown in FIG. 3.

When an aircraft passes through a water vapor cloud containing volcanicash, incoming air 302 containing water vapor and volcanic ash particles202 enters the engine 318. Some of the ash particles pass directlythrough, and become heated by, the jet engine of the aircraft. This hastwo effects. First, the water vapor in droplets having embedded ashparticles is boiled off due to the high temperature of jet enginecombustion, removing the ash particles from within the droplets andexposing the ash particles (or if embedded in ice crystals, the ice issimilarly melted and boiled off, exposing the particles of volcanicash). Second, the ash particles become heated. Thermal characteristicsof these heated ash particles allow detection by infrared (also referredto herein as “IR”) sensors configured to search for specificelectromagnetic radiation emissions in the infrared band of theelectromagnetic spectrum. As used herein, the terms “infrared emissions”and “infrared radiation” have the same meaning and refer toelectromagnetic radiation in the infrared band of the electromagneticspectrum. Jet exhaust 316 contains heated ash particles, heated sootparticles, and heated gasses. Soot particles are byproducts ofhydrocarbon combustion and consist of complex hydrocarbon molecules.Soot particles in the jet exhaust 316 are heated to a similartemperature as the ash particles.

A system having an infrared sensor directed at the exhaust of a jetengine can therefore be used to detect the presence of ash. Such asystem is depicted integrated with an airplane in FIG. 3 and as a blockdiagram in FIG. 4. Multiple infrared sensors are shown in FIG. 4. Theserepresent sensors that may be placed at different locations on anaircraft to view exhaust from different jet engines. If the system 304detects infrared radiation indicative of volcanic ash particles, then anotification signal is sent to an aircraft central control system 326,also referred to herein as “computer,” which can in turn inform thepilot that the aircraft has flown through a water vapor cloud containingvolcanic ash. The pilot may then take any necessary corrective measures,which may include throttling back the engines and alteration of theaircraft's course to avoid the volcanic ash. Because the system 304allows detection of ash prior to engine failure, flight through ash isavoided for a substantial time interval, thereby preventing additionaldamage by the ash to the aircraft parts.

The system comprises an infrared sensor 312 coupled to a location on anaircraft 100 from which jet engine 318 exhaust 316 may be opticallymonitored by the sensor 312. The sensor 312 may be a standard infraredsensor capable of withstanding conditions present on the externalsurface of an aircraft fuselage during flight—namely, high wind,vibration and low temperature, and should be able to discern infraredintensity and wavelengths accurately. In the embodiment depicted in FIG.3, jet engine 318 extends downward from wing 320, so that an infraredsensor 312 located on the underside 322 of wing 320 can be directed suchthat it views the exhaust 316 without being blocked by other portions ofthe aircraft 100. It should be recognized that a large variety ofaircraft jet engine configurations exist, and that placement of thesensor on the bottom side of a wing is simply an exemplary configurationwhich may be utilized in the aircraft jet engine configuration depictedin FIG. 3.

Infrared sensor 312 is coupled to a volcanic ash infrared signatureprocessing component 324 (also referred to herein as an “IR processor”).The IR processor 324 receives raw data from the IR sensor 312 andanalyzes the data to determine whether volcanic ash is present in thejet engine exhaust 316. The IR processor 324 may be coupled to anaircraft central control system/computer 326. The IR processor 324 maybe any component capable of analyzing the raw data provided by the IRsensor 312 and reasonably discerning the presence of volcanic ash fromthat raw data. Examples of such a component include a digital processorcoupled to digital memory containing instructions for analyzing the rawdata from the sensor in order to determine the presence of volcanic ash,or an analog circuit built to make required calculations. The IRprocessor 324 may be either a standalone, separate physical component ormay be a programmed set of instructions entered into a component of theaircraft that would be otherwise present.

The computer 326 can alert a pilot that ash is present through the useof a display 328, and the pilot may take appropriate action. Optionally,the computer 326 may automatically take action upon detection of ash,utilizing a hazard avoidance system 330. For example, if a severe levelof ash is detected, the system may automatically throttle back theengines and drop altitude, or may do so after prompting the pilot.Optionally, the computer 326 may relay information about the presence ofvolcanic ash to other aircraft or to ground based computer through theuse of communications system 332.

Preferably, infrared sensor 312 is oriented to maximize thediscernability of the IR signature generated from the heated volcanicash particles. All objects emit blackbody radiation with a spectral peakwavelength inversely proportional to their temperature. This includesatmospheric gasses, aircraft components, solid particulates suspended inair, such as ash and soot, as well as other objects. A “spectral peak,”for data consisting of a plot of wavelength versus intensity, is a localmaximum in intensity. A “spectral peak wavelength” is the wavelength atwhich that local maximum occurs. An ideal black body radiation curve foran ideal black body at a given temperature has a single spectral peakhaving a single spectral peak wavelength. In the temperature rangespresent on earth, all objects emit significant blackbody radiation inthe infrared band of the electromagnetic radiation spectrum. Therefore,an IR sensor directed at jet exhaust will detect IR radiation emittedfrom objects other than simply the heated ash particles. However, the IRemitted from ash particles can be distinguished from other normallydetected IR radiation in several ways.

Preferably, it should be ensured that the IR sensor 312 is pointed awayfrom any solid aircraft part, especially heated parts of the aircraft100 near the engine 318 and the engine 318 itself. This is because thesolid aircraft 100 parts such as the engine 318 emit a large amount ofIR radiation at wavelengths similar to that of the hot ash. If the IRsensor 312 is oriented such that it receives IR radiation from solidaircraft parts, the signal from the solid aircraft parts may “drown out”the signal from the ash, and it will be difficult or impossible todiscern an IR signal from the ash.

Therefore, preferably, infrared sensor 312 should be pointed to viewonly the exhaust 316 from the jet engine 318, such that IR sensor 312detects only IR emissions from a) the heated exhaust gasses which exitthe jet engine; b) any heated soot particles which exit the engine; c)any heated volcanic ash particles which exit the engine; and d) theunheated background atmosphere. Solid aircraft parts are excluded fromthis view. The IR sensor 312 may have a feature for limiting the scopeof view of the sensor IR. For example, a peripheral shield may be usedto block electromagnetic radiation from locations outside of a definedscope of vision. This may assist in blocking out emissions from solidaircraft components.

For the aircraft 100 depicted in FIG. 3, the infrared sensor 312 ispointed at an angle α in a direction that corresponds to roughly 10° to45° downward with respect to the length of the aircraft 100, and facingtowards the rear of the aircraft 100. This allows the sensor 312 toavoid viewing solid parts of the aircraft 100, while also allowing thesensor 312 to view exhaust 316 that retains sufficient heat from the jetengine 318. If the sensor 312 is pointed too far downward, there is arisk of picking up emissions from solid aircraft parts, while if thesensor is pointed too far backwards (e.g., less than approximately 10°downward with respect to the front-to-back axis), then it will not viewsufficiently heated aircraft exhaust 316. If the exhaust 316 viewed isnot sufficiently heated, then the signal-to-noise ratio of the IR signalfrom ash particles may be too low, in which case any emissions fromvolcanic ash particles may not be distinguishable over noise. Dependingon the position of the IR sensor 312 on the wing, the IR sensor may haveto be directed to the left or to the right in order to be facing the jetengine exhaust 316. When positioning the sensor on the wing, it shouldbe kept in mind that the sensor should be positioned to avoid viewingsolid aircraft parts.

With the IR sensor 312 pointed away from any solid aircraft parts, theIR signal from the ash may be distinguished from the IR emitted fromother emitters of a significant amount of blackbody radiation in thevicinity of the aircraft.

The surrounding atmosphere 334, which is unheated, emits a differentradiation pattern than that emitted by components of the heated jetexhaust 316, including heated gasses, volcanic ash and soot. Thetemperature of jet exhaust 316, including ash particles which exit fromjet engines 318 is typically around 900K to 1100K. Because all objectsemit black body radiation having a spectral peak wavelength inverselyproportional to their temperature, the temperature of the ash particlesproduces an IR pattern having a spectral peak different from that of themuch cooler surrounding atmosphere.

Unheated, surrounding atmospheric gasses 334 have a temperature thatvaries with altitude of the aircraft 100. For example, at 30,000 feetabove sea level, average temperature is approximately −47.83° F. or228.8K. At 35,000 feet, the temperature is approximately −65.61° F. or218.9222K. Spectral peak for gas at this temperature is approximately12.7 microns. This temperature is much lower than the temperature of jetexhaust 316 near the engine 318. Because the intensity of blackbodyradiation for any object at a given temperature drops off sharply forwavelengths substantially lower than the spectral peak wavelength of theblackbody radiation pattern for that temperature, the contribution ofblackbody radiation of the cool atmospheric gasses 334 to the emissionof the heated jet exhaust 316 at the spectral peak wavelength isvirtually zero. Emissions from heated jet exhaust 316 may therefore beeasily distinguished from surrounding atmosphere by simply examining theintensity of infrared radiation at or near the spectral peak wavelengthfor the exhaust emissions 316 and ignoring emissions near the spectralpeak wavelength for the cooled atmosphere. It should be recognized thatIR emissions need not be examined at the precise spectral peakwavelength for the exhaust emissions 316. A range of IR wavelengths or“spectral window” surrounding or near the precise spectral peakwavelength for the heated exhaust may be examined with similar result.

All components of the exhaust from the engines 316 are heated toapproximately the same temperature. This includes the exhaust gasses, aswell as soot particles and any volcanic ash particles present. Becausesolid particles are much stronger IR emitters than gasses, IR emissionsat the spectral peak wavelength for the heated exhaust 316 will bedominated by emissions from the soot particles and any volcanic ashparticles that are present. In order to determine whether there is anyvolcanic ash in these emissions, intensity of IR emissions at thespectral peak wavelength is constantly monitored to determine whetherthere is an anomalous rise corresponding to the presence of additionalsolid particulates—namely, volcanic ash particles. A sufficient rise inIR radiation at or near this spectral peak wavelength will almost alwaysindicate the presence of additional solid particulates in the form ofvolcanic ash particles. At cruise altitude in the stratosphere, volcanicash is virtually the only form of solid particulate present insufficient concentrations to cause such a rise in IR radiation. Neitherdust storms nor smoke from fires typically reach as high as cruisealtitude. A notable exception is smoke and dust from large nuclearexplosions or dust from dust storms within the troposphere. Such smokeand dust may also cause an anomalous rise in IR radiation and bedetected as volcanic ash by the system. However, because suchparticulates should be avoided by aircraft as well, detection by thesystem is beneficial.

A method for determining whether there is an anomalous rise in intensityat the spectral peak wavelength corresponding to heated volcanic ash ispresented in FIG. 5.

In step 500, the spectral peak wavelength for jet engine exhaust atpresent flight conditions is determined. This may be done by determiningthe temperature of the jet exhaust 316 at the location being examinedand applying Wien's displacement law to calculate the spectral peakwavelength (λ_(max)) for the given temperature. Wien's displacement lawallows calculation of the spectral peak wavelength (λ_(max)) for anideal black body at a given temperature. It states that λ_(max)=b/T,where T is the temperature of the ideal black body and b is Wien'sdisplacement constant, equal to 2.8977685×10⁻³ m·K. Using this law, thewavelength spectral peak of the black body radiation for jet exhaust at1000K can be calculated as 2.8977685×10⁻³ m·K/1000K=2.8977685×10⁻⁶m=approximately 3 microns. It would be desirable to monitor emissions atthis wavelength to detect the presence of volcanic ash. However, becausewater molecules in the atmosphere have a strong absorption peak fromabout 2.3 to 3.2 microns, it would be difficult to get a goodmeasurement of IR emissions from volcanic ash within that range.Therefore, a range of wavelengths outside of this water moleculeabsorption peak range is chosen to obtain a strong signal from volcanicash. A range of between 3.2 and 3.4 microns is an appropriate range tomonitor for this purpose.

In step 502, the “normal” or “steady state” intensity level for infraredemissions at or near the spectral peak wavelength for the exhaust isdetermined. This “steady state” intensity level is caused primarily bysoot exiting from the engines. It is therefore equivalent to theintensity of infrared radiation emitted by that soot. The amount of sootthat exits the engine, and therefore the “steady state” intensity level,may vary depending on the current flight conditions. However, an abruptrise in intensity, as opposed to a “steady” radiation intensity level,at constant or slowly changing flight conditions may indicate thepresence of additional IR emitters beyond the expected amount of soot.The speed of such a rise which may indicate an undesirable level ofvolcanic ash in the jet exhaust may be a user selectable parameter basedon how sensitive a user, such as a pilot or technician, wishes thesystem to be. Some such users would prefer more warnings, while otherswill tolerate fewer false alarms and will only wish to get warningsabout very serious encounters. Exemplary values for anomalous timeperiods for IR intensity increases are one minute for a relatively smallrise (for example, a 20% rise) or ten minutes for a large rise (forexample, a 100% rise). The presence of additional emitters in the formof volcanic ash would cause the intensity of the radiation emitted atthe spectral peak wavelength to increase by a certain factor (an“anomaly” factor) depending on the amount of ash in the exhaust 316.

In step 504, the intensity at this spectral peak wavelength is monitoredfor an increase above an intensity level threshold corresponding to anundesirable amount of volcanic ash. The intensity threshold may bedetermined in a number of ways.

In a first way, the threshold is dependent on the ratio of an elevatedconcentration of particulates (such as ash and soot), including anundesirable concentration of volcanic ash, to a normal concentration ofsolid particulates including only the normal level of soot. Becauseintensity of blackbody radiation emitted by solid particulates suspendedin air increases in proportion to the concentration of the solidparticulates, the intensity threshold can be calculated as a percentincrease over the steady state intensity level of IR at the spectralpeak wavelength as follows. In the calculations below, C_(a) isequivalent to an undesirable concentration of ash in the jet exhaust,C_(s) is equivalent to a normal concentration of soot in the jet engineexhaust, C_(p) is equivalent to C_(a)+C_(s) which is equivalent to thetotal particulate concentration including an undesirable amount of ash,I_(n) is equivalent to the “normal” or “steady state” intensity level atthe spectral peak wavelength, and I_(t) is equivalent to the intensitylevel threshold at the spectral peak wavelength which indicates anundesirable amount of volcanic ash in the jet engine exhaust. Thefollowing calculations begin with the assumption that the intensitylevel of the radiation at the spectral peak wavelength increases indirect proportion to any increase in concentration of solidparticulates. Therefore, the ratio of an increased concentration ofparticulates to the normal concentration of particulates is equivalentto the ratio of an increased intensity level to a normal intensitylevel.(C _(a) +C _(s))/C _(s) =I _(t) /I _(n)I _(t) =I _(n)(C _(a) +C _(s))/C _(s)I _(t) =C _(p) /C _(s) ×I _(n)

In other words, a threshold intensity level can be defined as the ratioof a total particulate concentration with undesirable amount of ash tothe “normal” particulate concentration times the normal intensity levelof infrared at the spectral peak wavelength. If this threshold intensitylevel is detected, then there is an undesirable amount of volcanic ashin the engine exhaust.

If desired, several threshold intensity levels, for example, I_(t1),I_(t2), may exist which correspond to different concentrations ofvolcanic ash, for example C_(a1), C_(a2), etc. Different warnings may begiven to the pilot at each concentration. For example, a firstconcentration may indicate the presence of a level of ash that mayrequire enhanced maintenance procedures after a flight while a second,higher concentration may indicate the need for repair or replacement ofparts after a flight.

Another method of determining a threshold intensity level is bydetermining the highest concentration of soot reasonably possible forgiven flight conditions, determining a corresponding intensity level (a“maximum intensity level”) of infrared emissions at the correspondingspectral peak wavelength, and determining an increment above which thereis reasonable confidence that an undesirable concentration of volcanicash must be present. The increment may be represented as a percentage.For example, an increment of 15% may be chosen such that the thresholdintensity level is equal to the maximum intensity level times 115%. Anincrement may also be chosen such that it is below the thresholdintensity level calculated using the ratio of undesired particulateconcentrations to normal particulate concentrations, as explained above.Thus if the ratio of undesired particulates to normal particulates is 2to 1 (therefore, a 100% increase), the increment may be chosen as 50%,which is less than 100%.

FIG. 6 depicts an estimation of blackbody radiation as detected byinfrared sensor for a number of different ash concentrations: no ash602, some ash 604, and a dangerous level of ash 606. Spectral peaks 608,610, 612 for each of the curves are shown. Each level of ash has a broadspectral curve. A window 614 is shown, which corresponds to a range ofapproximately 3.2 to 3.4 microns in wavelength. To determine anintensity level, the system may average the intensity levels measuredwithin this window 614. Example average intensity levels within thewindow 614 are depicted as horizontal lines in FIG. 6. The averageintensity level for the “some ash” curve 604 is line 616 and for the“dangerous ash” curve 606, the average intensity level is line 618. Theexample values shown in FIG. 6 for the three curves shown are 0.58arbitrary units for no ash, 0.68 arbitrary units for some ash and 1.48arbitrary units for a dangerous level of ash.

Example calculations will now be provided to illustrate the conceptsdiscussed above. It is generally accepted that soot is emitted from jetengines at a rate of 0.04 g per kilogram of fuel burned. This translatesto a concentration of soot equal to approximately 3.3×10⁻⁴ g/m³ at thenozzle. In other words, a standard concentration of soot in the jetengine exhaust for these numbers is approximately 3.3×10⁻⁴ g/m³. Anormal intensity level of IR radiation in the spectral window for thissoot is measured. This normal intensity level can be labeled a 100%intensity level and corresponds to the “no ash” curve 602 in FIG. 6.

Undesirable concentrations of ash are determined. In these examplecalculations, numbers from European flight guidelines for dangerousconcentrations of volcanic ash are provided. In Europe, the threshold ofash concentration at which aircraft must undergo enhanced maintenanceprocedures is 2×10⁻⁴ g/m³ and the threshold of ash concentration atwhich aircraft should not fly is 2×10⁻³ g/m³. Allowing for thermalexpansion of air heated in the engine, these values translate to 5×10⁻⁵g/m³ in the jet engine exhaust for enhanced maintenance procedures and5×10⁻⁴ g/m³ in the jet engine exhaust for the no-fly threshold.

The total undesirable concentration of particulates (soot plus ash) forthe enhanced maintenance procedures is equivalent to 3.3×10⁻⁴g/m³+5×10⁻⁵ g/m³=3.8×10⁻⁴ g/m³. The ratio of total undesirableparticulates to normal particulates is 3.8 to 3.3 which is equivalent to115%. Thus a first threshold may be set at 115% of normal IR emissions(shown as the first threshold line 616 in FIG. 6) or may be set to avalue between 100% and 115%. Observation of IR emissions in the spectralwindow which meet this threshold would indicate a concentration of ashrequiring enhanced maintenance procedures, which typically include morefrequent inspection of turbine blades and of hot surfaces inside jetengines.

The total undesirable concentration of particulates (soot plus ash) forthe no-fly threshold is equivalent to 3.3×10⁻⁴ g/m³+5×10⁻⁴ g/m³=8.3×10⁻⁴g/m³. The ratio of total undesirable particulates to normal particulatesis 8.3 to 3.3 which is equivalent to 250%. Thus a second threshold maybe set at 250% of normal IR emissions (depicted as the second thresholdline 618 in FIG. 6) or may be set to a value between 100% and 250%.Observation of IR emissions in the spectral window 614 which meet thisthreshold would indicate a concentration of ash indicating that anaircraft should not fly in that area.

FIGS. 7A-7D depict several other aircraft engine configurations andcorresponding exemplary sensor locations. In each of these figures, thesensor orientation is chosen in line with the principles describedabove—namely, the sensor should be oriented such that the sensor viewsjet engine exhaust in close proximity to the engine exit, whichtherefore retains significant heat but such that the sensor does notdetect IR radiation from solid aircraft parts.

FIGS. 7A-7B depict a twin-engine aircraft 700 in which each engine isenclosed within the fuselage. The exhaust 704 exits the engines at therear of the aircraft. Infrared sensors 702 may be placed at any locationon the aircraft at which exhaust is viewable while the solid aircraftbody is excluded from view. One location for sensors in thisconfiguration is at the tail of the aircraft. At this location, thesensors can be pointed downwards by approximately 10°-45° with respectto the length of the aircraft, and can be pointed to an angle ofslightly less than 10° to the left or right. In such an orientation,exhaust from the engines is viewable and solid aircraft components areexcluded from view.

FIGS. 7C-7D depicts a twinjet configuration aircraft 720 in which twoengines are attached to the fuselage to the rear of the wings and extendto the side of the aircraft 720. In such a configuration, an appropriatelocation for infrared sensors 722 would also be on the tail of theaircraft. At this location, sensors may be pointed downwards byapproximately 10°-45° with respect to the length of the aircraft, andcan be pointed straight downward by 90° with respect to the width of theaircraft.

Although the embodiments disclosed so far involve measurements made on asingle airplane, further embodiments use measurements made aboardmultiple aircraft, the measurement data being relayed to a ground-basedcentral processing site. The central processing site comprises acomputer that combines the measurements from all aircraft, together withlocations and times at which the measurements were made, meteorologicaldata, and information about plausible volcanic sites, to better estimatethe location and other characteristics of the ash plumes.

In particular, the central processing site may comprise a data fusionsystem that receives infrared emission measurements from multipleaircraft and combines them to form an estimate of a plume'scharacteristics and, optionally, construct a three-dimensional model ofthe plume. In this case, each of a plurality aircraft transmits arespective set of infrared emission measurements (and associatedmetadata, such as time and location of the aircraft) to a data fusioncenter via a network. More specifically, each aircraft comprises atransmitter and an antenna for wireless communication with the network.All measurements are incorporated into the data fusion system and areused to detect the presence of a volcanic plume and estimate the plume'scharacteristics. Optionally, the data fusion system also constructs athree-dimensional model of the plume. When the measured gasconcentrations from multiple aircraft indicate the presence of avolcanic plume, the data fusion system generates a warning to a humancontroller, e.g., a visual warning which is displayed by a controllerwarning display. The particular fusion algorithm or approach may vary.

There are several known hardware/software systems that combineobservations of phenomena from multiple mobile sensors to create abetter estimate than any single sensor could make on its own.

For example, meteorological measurements from diverse balloons andaircraft are transmitted via radio links and ground networks to aworkstation. The workstation runs a software program called 4DVAR, whichuses a variational cost-minimization approach to fuse data from multiplesensors at various times and places. (A technical summary of the 4DVARmethod can be found in the publication “Data assimilation concepts andmethods”, F. Boutier and P. Courtier, March 1999, MeteorologicalTraining Course Lecture Series, copyright ECMWF 2002, available athttp://www.ecmwf.int/newsevents/training/rcourse_notes/DATA_ASSIMILATION/ASSIM_CONCEPTS/Assim_concepts11.html,the contents of which are incorporated by reference herein in itsentirety.) Its output is an atmosphere model that is more accurate thanan analysis could produce from a lone sensor.

Another example is military radar. Military radar observations frommultiple ground and airborne radars are transmitted via various networksto a workstation. The workstation runs a Bayesian software model thatcombines evidence from various radar measurements to accurately track ahostile aircraft.

These methods are well known to persons skilled in the art of datafusion. Compared to the general meteorology case, in which manydifferent kinds of data are combined, the process of transmitting andcombining infrared emission measurements (a single kind of data) fromdifferent aircraft should not require undue experimentation by personsskilled in the art.

Installing and monitoring IR sensors in accordance with this disclosureon multiple aircraft that communicate with a network improves the chanceto detect and characterize a volcanic plume before it damages anyaircraft. A warning signal from the first aircraft to detect the plumecan be relayed to all aircraft in the area, even those without ash plumedetection systems.

In summary, the embodiments disclosed herein provide distinct advantagesas compared to prior solutions for detecting the presence of volcanicash, because no methods exist for detection of ash particles embedded inwater vapor clouds.

Furthermore, the embodiments disclosed herein provide direct warning toan airplane's pilot rather than relying on the process to issue a noticeto airmen.

While several exemplary embodiments have been disclosed, it will beunderstood by those skilled in the art that various changes may be madeand equivalents may be substituted for elements thereof withoutdeparting from the scope of this disclosure. In addition, manymodifications may be made to adapt a particular situation to theteachings of this disclosure without departing from the essential scopethereof. Therefore it is intended that the disclosure not be limited tothe particular embodiments disclosed for carrying out the teachings ofthis disclosure.

The invention claimed is:
 1. A method for detecting volcanic ashembedded in a water vapor cloud, comprising the following steps:measuring an intensity of infrared radiation emissions of a jet engineexhaust at or near a spectral peak wavelength for the jet engineexhaust; and generating a detection signal indicating the presence ofvolcanic ash in the jet engine exhaust when the intensity of infraredradiation emissions at or near the spectral peak wavelength exceeds athreshold.
 2. The method of claim 1, wherein the threshold is determinedby: determining a normal intensity of infrared radiation emissions atthe spectral peak wavelength during flight in which substantially novolcanic ash is present in the jet engine exhaust; identifying a normalconcentration of soot present in the jet engine exhaust; identifying afirst unwanted concentration of volcanic ash; calculating a totalunwanted particulate concentration by adding said normal concentrationto said first unwanted concentration; calculating a first concentrationratio by dividing said total unwanted particulate concentration by saidnormal concentration; and multiplying said normal intensity by saidfirst concentration ratio to determine said threshold.
 3. The method ofclaim 1, wherein the threshold is determined by: determining a normalintensity of infrared emissions at the spectral peak wavelength duringflight in which substantially no volcanic ash is present in the jetexhaust; identifying a normal concentration of soot present in the jetengine exhaust; identifying a first unwanted concentration of volcanicash; calculating a total unwanted particulate concentration by addingsaid normal concentration to said first unwanted concentration;calculating a first concentration ratio by dividing said total unwantedparticulate concentration by said normal concentration; and multiplyingsaid normal intensity by said first concentration ratio to determine amaximum intensity threshold; and setting said threshold to a value inbetween said normal intensity and said maximum intensity threshold. 4.The method of claim 1, further comprising: notifying a pilot when thedetection signal is generated.
 5. The method of claim 1, furthercomprising: automatically throttling back the engine when said detectionsignal is generated.
 6. The method of claim 1, further comprising:notifying a central processing site of the presence of volcanic ash whensaid detection signal is generated.
 7. The method of claim 1, wherein:measuring the intensity of infrared radiations emissions comprisescontinually monitoring the intensity of light emitted at or near adetermined spectral peak wavelength.
 8. The method of claim 1, furthercomprising: providing an infrared sensor; coupling said sensor to anaircraft at a location from which jet engine emissions are viewable; andorienting said infrared sensor such that substantially no solid parts ofthe aircraft are viewable by the sensor.
 9. A system for detectingvolcanic ash embedded in a water vapor cloud, comprising: an infraredsensor located and oriented to view jet engine exhaust; and an infrared(IR) processor receiving raw data from said infrared sensor anddetermining when an amount of volcanic ash above a threshold level ispresent.
 10. The system of claim 9, wherein said infrared sensor ismounted on an aircraft.
 11. The system of claim 10, wherein saidaircraft has a jet engine extending downwards from a wing, said infraredsensor is mounted on a bottom side of said wing, and said infraredsensor is pointed rearwards by an angle of between approximately 10° andapproximately 45° relative to the length of the aircraft.
 12. The systemof claim 9, wherein said IR processor is programmed to generate adetection signal when an intensity of infrared emissions at or near aspectral peak wavelength exceeds a threshold.
 13. The system of claim12, wherein said IR processor is programmed to determine said thresholdby: determining a maximum possible intensity of infrared emissions atthe spectral peak wavelength during flight in which substantially novolcanic ash is present in the jet exhaust; and multiplying said maximumpossible intensity by an increment to determine said threshold.
 14. Thesystem of claim 12, wherein said IR processor is programmed to determinesaid threshold by: determining a normal intensity of infrared emissionsat the spectral peak wavelength during flight in which substantially novolcanic ash is present in the jet exhaust; identifying a normalconcentration of soot present in the jet engine exhaust; identifying afirst unwanted concentration of volcanic ash; calculating a totalunwanted particulate concentration by adding said normal concentrationto said first unwanted concentration; calculating a first concentrationratio by dividing said total unwanted particulate concentration by saidnormal concentration; and multiplying said normal intensity by saidfirst concentration ratio to determine said threshold.
 15. The system ofclaim 12, wherein said IR processor is programmed to determine saidthreshold by: determining a normal intensity of infrared emissions atthe spectral peak wavelength during flight in which substantially novolcanic ash is present in the jet exhaust; identifying a normalconcentration of soot present in the jet engine exhaust; identifying afirst unwanted concentration of volcanic ash; calculating a totalunwanted particulate concentration by adding said normal concentrationto said first unwanted concentration; calculating a first concentrationratio by dividing said total unwanted particulate concentration by saidnormal concentration; and multiplying said normal intensity by saidfirst concentration ratio to determine a maximum intensity threshold;and setting said threshold to a value in between said normal intensityand said maximum intensity threshold.
 16. The system of claim 12,further comprising: a field of view limiting shield coupled to saidinfrared sensor.
 17. The system of claim 9, wherein: said IR processoris coupled to an aircraft control system.
 18. The system of claim 17,wherein: said aircraft control system is wirelessly linked to a centralprocessing site; and said aircraft control system sends a signal to saidcentral processing site when volcanic ash is detected in water vaporclouds.
 19. A method for detecting volcanic ash embedded in a watervapor cloud, comprising the following steps: determining a maximumpossible intensity of infrared radiation emissions from a jet engineexhaust at a spectral peak wavelength during flight in whichsubstantially no volcanic ash is present in the jet engine exhaust andmultiplying said maximum possible intensity by an increment to determinea threshold intensity; measuring an intensity of infrared radiationemissions of a jet engine exhaust at or near the spectral peakwavelength; and generating a detection signal indicating the presence ofvolcanic ash in the jet engine exhaust when the intensity of infraredradiation emissions at or near the spectral peak wavelength exceeds thethreshold.
 20. The method of claim 19, further comprising: automaticallythrottling back the jet engine when said detection signal is generated.21. The method of claim 19, further comprising: notifying a centralprocessing site of the presence of volcanic ash when said detectionsignal is generated.
 22. The method of claim 19, further comprising:coupling an infrared sensor to an aircraft in a position that permitsorienting the sensor to view the infrared radiation emissions of the jetengine exhaust and substantially no solid parts of the aircraft.